With specific reference to FIG. 1, a conventional three-tube cannula is shown and generally indicated by reference numeral 10. The three-tube cannula 10 includes a larger tube 12 to which a pair of smaller tubes 14 are attached. An accessory end 16 is connected to the larger tube 12 and the pair of smaller tubes 14. Guide wires or fixation pins (not shown) can be inserted into a pair of openings 18 and threaded through the pair of smaller tubes 14, so that the three-tube cannula 10 can be supported on the guide wires or the fixation pins. The tip of the three-tube cannula 10 is generally indicated by reference number 20 and is configured to be inserted into a patient through an incision (not shown).
The three-tube cannula 10 is typically made in an injection molding machine (not shown). It will be appreciated that during the forming process the larger tube 12 and the pair of smaller tubes 14 may be formed over tapered pins (not shown but known to one skilled in the art) to ensure the length, diameter and wall thickness of each of the tubes 12, 14. Another consideration is the relative alignment of the larger tube 12 and the pair of smaller tubes 14. More specifically, the larger tube 12 is defined by a center line 22 and the pair of smaller tubes 14 is defined by associated center lines 24. It will be appreciated that as the length of the pins in a typical injection molding machine increase in length, the ability to produce the larger tube 12 in parallel alignment to the pair of smaller tubes 14 becomes more difficult and expensive.
As with any manufacturing process, exact parallel alignment is not always required or attainable. With that said, deviations from absolutely parallel may cause the wall thickness of either the larger tube 12 or the pair of smaller tubes 14 to be too thin when compared to the same cannula manufactured with correct alignment. If a wall thickness 26 is too thin, quality issues arise and the rejection rate from the injection molding machine may become unacceptably high. Moreover, a thickness at a junction indicated by reference numeral 28 between the larger tube 12 and the pair of smaller tubes 14 must not be too thin as the possibility exists of poking the guide wire through one of the smaller tubes 14 into the larger tube 12.
One technique in forming the pair of smaller tubes 14 and/or the primary tube 12 over the pins in the injection molding machine requires gradually flaring one end of the pins, to make it possible to remove the three-tube cannula 10 from the injection molding machine. Flaring is defined as a slight gradual increase of the diameter of a pin at one end when compared to the other end. Because the pins of the injection molding machine are flared or tapered, it necessarily means that one end of the smaller tubes 14 and/or the larger tube 12 will have a larger diameter when compared to an opposite end. This change in diameter necessarily means that the guide wires or fixation pins inserted into the smaller tubes 14 will not be constrained as well when compared to one of the smaller tubes 14 having a constant diameter throughout. When the guide wires are removed or become free from the smaller tubes 14 the guide wires tend to fall away from the three tube cannula 10 as no structure of the cannula retains the guide wires.